The present disclosure relates to a casting roll for a twin-roll strip caster for manufacturing a strip directly from molten metal, and more particularly, to a casting roll for a twin-roll strip caster for preventing surface defects in a strip.
In general, according to a strip casting method using a twin-roll strip caster, a thin strip having a thickness of about 2 mm to about 6 mm can be formed by directly casting molten steel between two rotating rolls from a tundish, through a nozzle. Therefore, manufacturing costs may be decreased owing to reduced manufacturing processes, and product quality may be improved owing to rapid cooling. Thus, the strip casting method has been intensively researched as a steel forming process method.
FIG. 1 is a schematic view illustrating a twin-roll strip caster. Referring to FIG. 1, molten steel is supplied to a sump 4 from a ladle 1 through a tundish 2 and a submerged nozzle 3. The molten steel supplied to the sump 4 is formed into a strip while passing between twin casting rolls 5.
If the molten steel contained in a region of the sump 4, surrounded by the twin casting rolls 5 and edge dams 6 is exposed to the air, the molten steel may be oxidized, and resulting oxides may have a significant effect on product quality. Therefore, a meniscus shield 7 is disposed above the sump 4 to cover the surface of the molten steel with a gas atmosphere.
Thus, a space surrounded by the surface of the molten steel, the twin casting rolls 5, and the edge dams 6 is defined in an upper region of the sump 4, and gas may be supplied to the upper region of the sump 4 to form a gas atmosphere preventing oxidation of the molten steel.
The molten steel supplied to the interior of the sump 4 is solidified as shells while being cooled on the surfaces of the twin casting rolls 5, and the solidified shells are attached together as a strip at the nip of the twin casting rolls 5. In this case, since the molten steel is formed into solidified shells while quickly making contact with the twin casting rolls 5 and is then formed into a strip while leaving the twin casting rolls 5, the surface of the strip is subjected to sudden thermal stress during solidification.
Therefore, if the surfaces of the twin casting rolls 5 are flat, the solidified shells may be locally separated from the surfaces of the twin casting rolls 5 by the gas atmosphere (a separation phenomenon) and thus may be unevenly solidified due to non-uniform heat transfer. This may cause the formation of cracks in the solidified shells.
In the related art, a twin-roll strip caster in which dimples are formed in the surfaces of casting rolls is used for general steels to prevent the separation phenomenon of solidified shells. In addition, since the separation phenomenon of solidified shells occurs severely in the case of steels containing large amounts of nitrogen and manganese or steels undergoing a high degree of phase transformation during solidification, casting rolls of a twin-roll strip caster are shot-blasted to form dimples thereon (a shot blasting method), and fine grooves are formed in a parallel strip pattern in the circumferential direction of the casting rolls to discharge gas therethrough as illustrated in FIG. 2A.
However, if fine grooves are formed in the surfaces of casting rolls as illustrated in FIG. 2A, the shape of the fine grooves is transferred to a strip, and thus a striped pattern is present on the surface of the strip after a casting process, as illustrated in FIG. 2B. In addition, if the strip is cold-rolled and then subjected to a formation process (drawing process), the surface of the strip (product) may be heavily dimpled as illustrated in FIG. 3A. As illustrated in FIG. 3B, dimples are periodically formed in the surface of the strip after casting. Such dimples have a negative effect on the appearance of the strip and incur additional costs because an additional process such as a polishing process is necessary to remove the dimples.
As described above, if fine grooves are formed across the entire widths of casting rolls, during casting, locally excessively solidified shells (hereinafter referred to as skulls) may be mixed and pass through the nip of the casting rolls, and at this time, the solidified shells may be over-pressed and caught in the fine grooves. That is, the solidified shells may stick to the casting rolls (a sticking phenomenon).
As illustrated in FIGS. 6A and 6B, widthwise edge portions may be separated from the strip due to the sticking phenomenon, and the separated edge portions may stick to the casting rolls and rotate together therewith. Then, the separated edge portions may be remixed with molten steel, and thus more edge portions may be separated from the strip. As a result, a casting process may be suspended due to the introduction of separated portions.